Irregular Sea Research Articles

Dynamic analysis of monopile supported offshore wind turbines

This paper describes the stochastic dynamic response of National Renewable Energy Laboratory 5 MW offshore fixed-based wind turbine (OWT) under various soil conditions – medium dense sand, stiff clay and layered profiles in 20 m depth of water. The aerodynamic and hydrodynamic OWT loads are derived using the force-controlled approach. Usually the OWT generates power in an operational regime and survives at extreme wind speeds. Therefore, two met-ocean conditions adhering to the irregular Joint North Sea Wave Project spectrum are considered – one in an operational regime (average wind speed Vw = 12 m/s, significant wave height Hs = 4 m and peak spectral period Tp = 10 s) and another in a near cut-out regime (Vw = 22 m/s, Hs = 10 m, Tp = 14 s). The soil is modelled by way of a non-linear ground-to-spring model. For each sea state, time domain stochastic responses are calculated and the ensemble average response is calculated from 50 Monte-Carlo simulations. The change in ensemble average response due to changes in pile penetration depth and diameter of the piles for the three soil configurations is described. Results indicate that the dynamic response mainly depends on the stiffness of the soil and reiterate the need for detailed site-specific geotechnical investigations before designing OWT foundations.

Experimental and numerical investigations on the intact and damage survivability of a floating–moored oscillating water column device

This paper investigates the intact and damage survivability of a floating–moored Oscillating Water Column (OWC) device using physical model experiments and Computational Fluid Dynamics (CFD) simulations. Different extreme wave conditions have been tested using irregular and regular wave conditions. The device was moored to the tank floor via four vertical taut lines and the effect of the mooring line pre–tension on the device response was studied. It was found that the instantaneous position of the floating device was a key factor in the survivability analysis such that a certain irregular wave train that might not include the largest wave could induce the maximum response. Reducing the pre–tension minimized the maximum surge, but significantly increased the maximum tension due to mooring slack events causing snatch loads. A design regular wave with a period equal to the peak period and a height of 1.9–2.0 times the significant wave height could reasonably predict the same maximum line tension as the irregular sea state, but a smaller wave height was required to achieve the maximum surge. A single failure in the mooring system increased the maximum tension by 1.55 times the intact tension. For a damaged mooring system, using the same design regular wave condition derived from the survivability analysis with an intact mooring system could result in overestimating the maximum tension by more than 20% in comparison to the tension from the irregular sea state, but a smaller regular wave height or a different regular wave condition representing another sea state could lead to the same maximum tension. This highlighted the importance of investigating survival conditions with a damaged mooring system instead of simply using the same conditions derived for the intact mooring system.

Multiple-feedback control of power output and platform pitching motion for a floating offshore wind turbine-generator system

Abstract An approach for multiple-feedback control of generator power and nacelle fore-aft speed employing blade pitch and generator torque manipulations was developed for a spar-type floating offshore wind turbine-generator system. The development of this approach was carried out through numerical analysis using the aeroelastic simulation model (FAST), measured high wind speed data, and simulated irregular sea waves. The novelty of this multiple-feedback control approach is the increase of the generator torque in response to the platform pitching motion to the leeward side and the control of the generator power instead of the rotational speed. First, theoretical analysis of the open loop transfer function revealed that the multiple-feedback control approach improves the stability of the generator power control in the floating offshore wind turbine-generator system. Then, the optimal parameter settings of the multiple-feedback control were determined and the effectiveness of employing a first-order lag filter for the nacelle fore-aft speed was shown through a sensitivity analysis. A comparison of system performances demonstrated that the multiple-feedback control approach reduces the power output fluctuation, the platform pitching motion, and the damage equivalent fatigue load of the fore-aft bending moment at the tower-base part by 52%, 22%, and 16%, respectively, relative to the gain-scheduled feedback control of the rotational speed.

A Floating Platform with Embedded Wave Energy Harvesting Arrays in Regular and Irregular Seas

This paper presents a study on a cost-effective engineering model that integrates an array of floating wave energy converters with a vast platform, a viable option for multi-functional performance in renewable energy capture and ocean space utilization. The wave energy converters are floating buoyance columns flexibly connected with the elastic platform. Hydrodynamic interactions among the columns are analyzed using an exact matrix transform method based on linear wave theory in the frequency domain. A parametric governing equation of compounded wave energy converter referred to as a wave farm is formulated by using Hamilton’s principle which can be discretized using the Galerkin method. The effects of wave conditions and the parameters of hydraulic power take-off (PTO) on the wave energy absorption and dynamic characteristics of the energy harvesting system are investigated. Furthermore, the wave energy capture on irregular waves is also discussed. This research work aims at providing a theoretical guidance for wave energy harvesting system design.

Homoclinic Phenomena in Ship Motions

In the article will be reviewed the changes of ship motion patterns originating from a special class of phase space phenomena identified in the nonlinear dynamics literature as homoclinic (When the term appears in italic, it will imply truly homoclinic as well as heteroclinic events). The key incident is the contact of manifolds of saddle-type (i.e., unstable) objects, which triggers a global change in the phase space flow of the dynamical system. Homoclinic phenomena are more likely to appear in extreme dynamic behavior where, usually, the effect of nonlinearity is large. Their presence can be felt through the sudden termination of oscillatory motions; or, through the emergence of motion irregularity accompanied by loss of system robustness to external excitations. Homoclinic phenomena have been identified to lurk behind some well-known manifestations of ship dynamic instability. All known ship-related cases will be reviewed and explained in the current article. Analytical and numerical tools that can be used for predicting such phenomena effectively will be discussed, as well as, practical assessment methods that are suitable for evaluating ship vulnerability. Three types of ship motion will be reviewedrolling in beam seas;surf-riding and broaching-to in following/quartering seas; anddrifting and yawing in wind. For the roll motions, the investigation will be escalated from free rolling with bias, to resonant (periodically forced) rolling and the mechanism of capsize in beam seas through safe basin erosion; to stochastic rolling and the consideration of wave grouping excitations. In astern seas, the most profound homoclinic event is the transition from ordinary surging to surf-riding. It will be explained, initially, in a deterministic context through simple dynamical analogues. Differences arising from the level of detail in the mathematical model will be discussed (many degrees of freedom, memory effect, and nonlinear waves). Then, a multifrequency wave environment will be introduced, rendering the phase space flow time-varying and incurring significant qualitative effects. The monitoring of Lagrangian structures appearing in such unsteady phase space flows can offer unique insights about nonlinear ship motions and their interactions. Wave celerity is known to correlate with the occurrence of surf-riding in regular seas, although for irregular seas, its role has been uncertain. A phenomenological approach will be reviewed for evaluating this connection. Two possible methods for celerity's calculation in irregular seas will be presented. The third area where homoclinic phenomena have been identified is planar ship motions under strong wind loads. Two particular cases will be addressed: the transition from a course-keeping pattern to a turning motion pattern as the rudder angle is varied; and phenomena of "head wind" ship operation, under sluggish rudder control. The article will conclude with some thoughts as to the future direction in the field.

A New Species of Sexually Dimorphic Brittle Star of the Genus Ophiodaphne (Echinodermata: Ophiuroidea)

We describe a new species of sexually dimorphic brittle star, Ophiodaphne spinosa, from Japan associated with the irregular sea urchin, Clypeaster japonicus based on its external morphology, and phylogenetic analyses of mitochondrial COI (cytochrome c oxidase subunit I). Females of this new species of Ophiodaphne are characterized mainly by the presence of wavy grooves on the surface of the radial shields, needle-like thorns on the oral skeletal jaw structures, and a low length-to-width ratio of the jaw angle in comparison with those of type specimens of its Ophiodaphne congeners: O. scripta, O. materna, and O. formata. A tabular key to the species characteristics of Ophiodaphne is provided. Phylogenetic analyses indicate that the new species of Ophiodaphne, O. scripta, and O. formata are monophyletic. Our results indicate that the Japanese Ophiodaphne include both the new species and O. scripta, and that there are four Ophiodaphne species of sexually dimorphic brittle stars with androphorous habit.

Gyroscopic power take-off wave energy point absorber in irregular sea states

The Gyroscopic power take-off (GyroPTO) wave energy point absorber has the operational principle somewhat similar to the so-called gyroscopic hand wrist exerciser. Inside the float of the GyroPTO, there is a mechanical system made up of a spinning flywheel with its spin axis in rolling contact to a ring. At certain conditions, the ring starts to rotate at a frequency equal to the peak angular frequency of the wave excitation. In this synchronized state, the flywheel is running at almost constant speed, so the generated power from the generator becomes constant as well. In this paper, the performance of GyroPTO in irregular sea waves is investigated. To improve the stability (synchronization) margin of the device, a magnetic coupling mechanism has been added between the spin axis and the flywheel, which also makes the semi-active control of the device possible. Theoretical modeling of the GyroPTO is carried out using analytical rigid body dynamics, and a 4-DOF nonlinear model is established. Further, linear wave theory has been applied to calculate the hydrodynamic moments acting on the float. Rational approximation is performed on the frequency response function of the radiation damping moments, leading to an extended state vector formulation of the coupled structure- wave system. Simulation results show that magnetic coupling successfully improves the stability of the flywheel in irregular sea states. With given significant wave height and peak frequency, it is shown that the synchronization of the device is more easily obtained in narrow-banded wave than in broad-banded wave. As a result, larger values of generator gain can be chosen in narrow-banded waves, leading to larger power output. Making use of both the generator and the magnetic coupling, semi-active control algorithm might further improve the performance of the GyroPTO in real sea.

Control of the CCell Oscillating Surge Wave Energy Converter

CCell is a type of Oscillating Surge Wave Energy Converter (OSWEC) under development by Zyba Renewables Ltd. Laboratory scale wave tank tests have been conducted with a prototype device equipped with a passive controllable hydraulic power take-off system. The results are used to derive a simplified semi-analytical model of the governing hydrodynamics, providing a basis for simulation studies of control algorithms. The Simple and Effective control strategy of Fusco and Ringwood (2013) is used to generate an angular velocity trajectory designed to maximise energy capture whilst keeping the device motion within physical displacement constraints. Velocity tracking is achieved using a State Feedback Control architecture with integral action (SFCIA), combined with an integral position controller acting on measured angular position in order to reduce drift effects. Good performance when subjected to scaled real irregular sea states is observed in terms of energy capture and motion, and good robustness to parameter variation is demonstrated.

Estimation of added resistance and ship speed loss in a seaway

The prediction of the added resistance and attainable ship speed under actual weather conditions is essential to evaluate the true ship performance in operating conditions and assess environmental impact. In this study, a reliable methodology is proposed to estimate the ship speed loss of the S175 container ship in specific sea conditions of wind and waves. Firstly, the numerical simulations are performed to predict the added resistance and ship motions in regular head and oblique seas using three different methods; a 2-D and 3-D potential flow method and a Computational Fluid Dynamics (CFD) with an Unsteady Reynolds-Averaged Navier-Stokes (URANS) approach. Simulations of various wave conditions are compared with the available experimental data and these are used in a validation study. Secondly, following the validation study in regular waves, the ship speed loss is estimated using the developed methodology by calculating the resistance in calm water and the added resistance due to wind and irregular waves, taking into account relevant wave parameters and wind speed corresponding to the Beaufort scale, and results are compared with simulation results obtained by other researchers. Finally, the effect of the variation in ship speed and therefore the ship speed loss is investigated. This study shows the capabilities of the 2-D and 3-D potential methods and CFD to calculate the added resistance and ship motions in regular waves in various wave headings. It also demonstrates that the proposed methodology can estimate the impacts on the ship operating speed and the required sea margin in irregular seas.

A model of the received signal for shipborne inverse synthetic aperture radar together with an error analysis

ABSTRACTIn shipborne inverse synthetic aperture radar (ISAR), not only the target, but also the radar ship will have nonstationary 3-dimensional rotations induced by the irregular sea wave. Thus, how to compensate the radar ship’s rotations is the key problem in reconstructing the shipborne ISAR image of ship target successfully. In this letter, a novel received signal model of shipborne ISAR is proposed. On the basis of the proposed model, the Doppler frequency induced by the rotations of the target and the radar ship can be separated from the received signal, individually. An analysis of the effects of the radar ship’s rotations on the obtained image is given. Error analysis and imaging simulation are done and the results verified the correctness of the proposed model and the effects of the radar ship’s rotations.

Green water loads on FPSOs exposed to beam and quartering seas, part I: Experimental tests

The green water on FPSOs subjected to beam and quartering irregular seas is evaluated through a combination of ocean basin model tests and CFD simulations. Part I provides an exploration of the extensive experimental outcomes, while critical water on deck events are simulated in Part II using CFD. Based on comprehensive model test data, many aspects of green water resulting from the interaction of beam and quartering waves with an FPSO are explored. The model test was performed for a barge type FPSO, with measurements of water elevations and loads during green water events. The wave parameters were based on Brazilian waters metocean data and load conditions were chosen to maximize the occurrence of water on deck events. Considering the large number of monitored green water events, identification of critical regions near the deck edge and the water on deck propagation are characterized, including the influence of a riser balcony along the side hull. Additionally, an alternative formulation for maximum load estimation on small obstacles near the deck edge based on elevations is developed through a simplified local impact analysis. The results are compared with experimentally measured loads, revealing itself as a less conservative approach for green water loads determination.

Investigation of Sea Surface Effect on Shallow Water Reverberation by Coupled Mode Method

The effects of rough sea surface on the long-range bottom reverberation in shallow seas are studied by the coupled mode reverberation theory. The scattering effect caused by irregular rough sea surface is described by couple coefficients. The decaying rules of long-range bottom reverberation level are simulated at different sea states, and the rough sea surface effect on the coherence of distant bottom reverberation is also discussed. It is indicated that irregular upper boundary has changed the propagation effect of the shallow water waveguide, and bottom reverberation, which is dominated among other kinds of reverberation in shallow water, is affected by the sea surface scattering as the increasing sea state. Compared with other literatures, the emphasis of this paper is to present the mechanism of rough sea surface scattering by describing the transfer of energy between different modes, and the details of energy transitions between different modes which are caused by sea surface scattering are presented for different sea states. With the increasing sea state, stronger mode coupling caused by surface scattering would affect the intensity and its space coherence of bottom reverberation obviously.

Echinoderm ichnology: bioturbation, bioerosion and related processes

AbstractAmong invertebrates and both in modern and ancient marine environments, certain echinoderms have been and are some of the most active and widespread bioturbators and bioeroders. Bioturbation and/or bioerosion of regular and irregular echinoids, starfish, brittle stars, sea cucumbers and crinoids are known from modern settings, and some of the resulting traces have their counterparts in the fossil record. By contrast, surficial trails or trackways produced by other modern echinoderms, e.g., sand dollars, exhibit a lower preservation rate and have not yet been identified in the fossil record. In addition, the unique features of the echinoderm skeleton (e.g., composition, rapid growth, multi-element architecture, etc.) may promote the production of related traces produced by the reutilization of echinoderm ossicles (e.g., burrow lining), predation (e.g., borings), or parasitism (e.g., swellings or cysts). Finally, the skeletal robustness of some echinoids may promote their post mortum use as benthic islands for the settlement of hard-substrate dwellers.

Calculation of Ship Wave Bending Moment in Irregular Seas

Calculation of Ship Wave Bending Moment in Irregular Seas

Control of Wave Energy Converters for Maximum Power Absorption with Time Domain Analysis

A discrete control of latching is used to increase the bandwidth of the efficiency of the Wave Energy Converters (WEC) in regular and irregular seas. When latching control applied to WEC it increases the amplitude of the motion as well as absorbed power. It is assumed that the exciting force is known in the close future and that body is hold in position during the latching time. A heaving vertical-cylinder as a point-absorber WEC is used for the numerical prediction of the different parameters. The absorbed maximum power from the sea is achieved with a three-dimensional panel method using Neumann-Kelvin approximation in which the exact initial-boundary-value problem is linearized about a uniform flow, and recast as an integral equation using the transient free-surface Green function. The calculated response amplitude operator, absorbed power, relative capture width, and efficiency of vertical-cylinder compared with analytical results.

Human Smuggling and Intentions to Migrate: Global Evidence from a Supply Shock Along Africa-to-Europe Migration Routes

Africa-to-Europe irregular migration depends heavily on human smuggling services. The demise of the Gaddafi regime in 2011 marked the end of a bilateral agreement between Italy and Libya and opened the Central Mediterranean Route for irregular border crossing. How did this remarkable increase in human smuggling services affect migration intentions in the rest of the region? This paper isolates a causal impact by exploiting the spatial dimension of the smuggling network and its change over time, which produced a heterogeneous decrease in bilateral migration distances between countries in Africa and Europe. We use this source of variation and a novel dataset of bilateral distances along irregular land and sea routes, combined with cross-country survey data on individual intentions to move from Africa to Europe between 2010 and 2012. Netting out pair- and country-by-time-specific fixed effects, we find a large negative effect of distance along smuggling routes on individual migration intentions. Shorter distances increase the willingness to migrate especially for youth, (medium) skilled individuals and those with a network abroad. The effect is stronger in origin countries not too far from Libya and with weak rule of law.

Simulation-Based Analysis of Ship Motions in Short-Crested Irregular Seas

Simulation-Based Analysis of Ship Motions in Short-Crested Irregular Seas

Sensitivity Analysis of Material and Load Parameters to Fatigue Stresses of an Offshore Wind Turbine Monopile Substructure

Steel monopiles are support structures mostly applied for offshore wind turbines. Their installation is straightforward, in particular, in shallow and medium waters. While the wind turbine tower is primarily affected by wind, the wave loads are dominant for the monopile, as it is submerged to a large extent. This study deals with the influence of uncertainties in material and load parameters on the behaviour of those structures. It is investigated how the scattering of material properties (namely Young’s modulus of elasticity) affect the structural response. In addition, loads with different characteristics are applied, and it is examined how the changes in loads influence the structural response. The analysed output data of interest are the extreme stresses leading to the accumulation of fatigue damage. In order for a realistic modelling, wave loads are considered with irregular sea states with different wave characteristics (significant wave heights and wave peak periods). The final aim of the analysis is to classify the effects of specific wave characteristics on the stresses by means of a sensitivity analysis. The analysis shows that variations in the wave peak period have the strongest influence on stress outputs. This effect results from the strong sensitivity of the structural dynamical response to the decrease of the difference between the values of the wave peak frequency and the natural frequencies of the structure.

Point Absorber Wave Energy Converters in Regular and Irregular Waves with Time Domain Analysis

A discrete control of latching is used to increase the bandwidth of the efficiency of the Wave Energy Converters (WEC) in regular and irregular seas. When latching control applied to WEC it increases the amplitude of the motion as well as absorbed power. It is assumed that the exciting force is known in the close future and that body is hold in position during the latching time. A heaving vertical-cylinder as a point-absorber WEC is used for the numerical prediction of the different parameters. The absorbed maximum power from the sea is achieved with a three-dimensional panel method using Neumann-Kelvin approximation in which the exact initial-boundary-value problem is linearized about a uniform flow, and recast as an integral equation using the transient free-surface Green function.The calculated response amplitude operator, absorbed power, relative capture width, and efficiency of vertical-cylinder compared with analytical results.

Green water loads on FPSOs exposed to beam and quartering seas, Part II: CFD simulations

Following the experimental results described in Part I, this paper presents the development of a methodology for detailed CFD simulation of green water events. Based on model test data, extreme green water events resulting from beam and quartering waves are selected and the whole wave vessel interaction is simulated, including the water on deck propagation. The commercial CFD code ANSYS FLUENT® was customized to have boundary conditions to represent specific wave intervals from irregular sea state realizations and vessel movements prescribed based on experiments. By using a VOF-multiphase and SST-turbulent setup, the accuracy of 2D and 3D simulations are evaluated by comparison with experimental data in terms of free surface evolution and loads, with good agreement in most cases. Some deviations on the loads were observed, but they are coherent with the complex local flow conditions of typical impact flows due to effects such as wave breaking and air entrapment. The methodology presented in this paper gives access to high spatial resolution free surface position, water velocities and load distribution, aspects usually not available from experiments, which leads to new possibilities in the investigation of shadowing effects and dynamic loads during green water and wave impact extreme events.